Sound reproduction equipment still does not provide the full dimensionality and realism of the fine auditorium or concert hall, namely they do not fully increase the dimension capability of reproduction--reflections, as well as emphasizing harmonics. Many technical factors influence the sound of a concert hall: total reverberation time, the fading rate of the echoes, resonances, the absorptive and reflective properties of building materials, and the directional patterns of sound reflections are but the main considerations. This characterizes the acoustic property of location to the listener for which prior attempts to achieve have been made mainly in the recording process using phase shifting and increased high fidelity and other "surround sound" effects.
Recent attempts to recreate the sound field close to that which is heard at a live performance have merely delayed versions of the original signals and reproduced them through conventional speakers. The range of these reverberation delays have been quite short, typically approximating one second and the interval between reflections have not generally occurred at a large enough number of random times to produce a high density of echoes. One difficulty has been to create a reverberation system which has a relatively long reverberation time without having an undesirably long time between discrete echoes. To avoid the single echo effect, different cross-channel recycling loop techniques have also been used which produce audio output signals containing delayed signals which decay exponentially or logarithmically in amplitude.
Sound energy may be absorbed and reflected to a different degree by a particular material if the frequency of the incident wave varies. As the sound wave undergoes second, third and successive reflections, its spectral deviation from the original direct wave becomes greater since a spectral change occurs upon each reflection. Reinforcements and cancellations at different frequencies will also occur. The spectral content of the reflected wave will not retain the same relationship of the fundamental frequency of a sonic event with or among its harmonics. Successive reflections of a sonic event at some points in time may be louder at a listener's location than some or all of its predecessors arising from the same sonic event. Previously designed audio systems have not provided for these changing spectral relationships during decay, namely the subjective clarity characteristic of sharp transients and the mellowness associated with the relatively more rapid decrease of higher harmonics as opposed to the fundamental and lower harmonics of musical notes. Additionally, these systems show insufficient understanding of actual sound fields in auditoriums, concert halls and/or cathedrals, by restricting bandwidth, poorly choosing initial delays, by shortening the range of the maximum reverberation time achieved and by failing to consider the energy flux distribution as a function of the angle of incidence upon the listener. Satisfactory means to measure and characterize these relationships and to serve as design parameters and performance criteria for sound reproduction systems have not been developed sufficiently.
Many variations have been derived of the "matrixtype" four channel sound system, where four signals are generated. The front channel signals are identical or very similar to the conventional stereo signals. The rear channel signals are derived by obtaining a signal corresponding to the instantaneous voltage difference between the two stereo signals and remixing them with some portion of the front channels. To reproduce the signals, four conventional amplifiers drive respective loudspeaker systems. The use of only two additional channels have been obviously found inadequate to date partially explaining the multiplicity of competitive systems using variations in parameters. To encode and decode four channels independently have not been satisfactory. The rear speakers cannot possibly generate the diffuse sound field created by typical acoustic environments in auditoriums.